Buying a campervan in Perth (mediation?)

I am travelling to perth in mid ocober, and am planning to buy a small campervan there (ford econovan or similar).

I have found sites like gumtree, but they are people 2 people sales. And I would like to know if there perhaps is a mediation company in Perth? eg. that I pay to buy a set van, and bring it to a garage for servicing, so it will be ready upon arrival in Perth?

Since we are requiring a van that can legally seat 4, the amount availble online is quite few. We would hate to be stuck for weeks trying to find a suitable van.

Any suggestions?